The invention concerns a magnet arrangement comprising a superconducting magnet coil system which has an ohmic resistance of zero or more during operation, and a flux pump which comprises at least one superconducting switch and at least two superconducting secondary coils.
A magnet arrangement of this type comprising a superconducting magnet coil system is described by T. P. Bernart et al., Rev. Sci. Instrum., Vol. 46, No. 5, May 1975, pages 582–585.
The superconducting magnet coil system comprises one or more magnet coils which are connected in series and form a closed superconducting circuit. The superconducting magnet coil system is typically disposed in a cryostat. It may have an ohmic resistance of more than zero during operation if the superconductors used are charged to a value just below the critical current or if they do not show a clear transition between the superconducting and the normal conducting states. The principle of a flux pump consists in compensating resistive losses of the magnet coil through inductive injection of energy, or in charging or discharging the coil without requiring introduction of large currents into the cryostat. The invention concerns, in particular, superconducting magnet coil systems comprising a flux pump which has at least one superconducting switch and at least two superconducting secondary coils in which a voltage may be inductively established. To be able to feed this voltage into the superconducting magnet coil system for compensating resistive losses or for charging or discharging, the secondary coils must be superconductingly connected in series with the magnet coil system as may be effected e.g. through closing of a superconducting switch.
A magnet arrangement with a flux pump which comprises at least two superconducting secondary coils, is disclosed in T. P. Bernat et. Al., Rev. Sci. Instrum., Vol. 46, No. 5, May 1975 and from L. J. M. van de Klundert et al. Al., Cryogenics, May 1981. This flux pump is based on the fact that the superconducting magnet coil system is bridged by two current paths, which each comprise one switch and one superconducting secondary coil. Current is cyclically introduced into and discharged from a primary coil whose inductive coupling is equal and opposite to the secondary coils. If the superconducting switches which are connected in series with the secondary coils, are alternately opened and closed in a same cycle, a voltage is generated across the magnet coil system which is constant throughout the entire cycle, except for voltage peaks during opening of the switches.
Flux pumps are typically used for charging and discharging superconducting magnet coil systems. The advantage compared to direct feeding of the operational current into the coils consists in that the currents for operating the flux pump are much weaker than the typical magnet currents. The current feed lines may thereby be reduced in size and the heat input into the cryostat may be decreased.
Superconducting magnets are also used for applications for which the magnet coils remain at field for years after the charging process and should have a minimum field drift. This includes, in particular, superconducting magnet coil systems for magnetic resonance methods. For such magnet systems, the use of a flux pump is not of primary interest for charging the magnet system, rather for stabilization of the magnetic field during operation. An efficient flux pump would provide various advantages in this respect. Magnets comprising partial coils of high-temperature superconductors may e.g. be constructed which do not meet the currently conventional drift specifications for magnetic resonance applications without additional measures. This would permit construction of magnets with fields which are stronger than the conventional fields today. Moreover, the use of a flux pump to stabilize the field could increase the load on the superconductors in the magnet which would permit construction of more compact and less expensive magnets.
Conventional flux pumps are not suited to be used for precise field stabilization over long time periods. Voltage peaks across the magnet coil system occur during opening of superconducting switches which cannot be tolerated for sensitive applications such as magnetic resonance methods. Moreover, at least one superconducting switch must be opened in each phase of the pump cycle to permit feeding of the voltage induced in the secondary coil into the magnet coil system. Heat is thereby generated in conventional switches which produces large losses in cooling liquid in the cryostat. The thermal stability in the cryostat is also very important for the stability of the field. For sensitive applications, such as magnetic resonance methods, the heat input into the cryostat must therefore be minimized.
It is the object of the present invention to improve a flux pump in accordance with prior art in such a manner that, in addition to charging and discharging of a superconducting magnet coil system, good long-term stabilization of the magnetic field of the magnet coil system during operation is also possible, in particular, when the magnet coil system is slightly resistive and the requirements for the field stability are very high. A particular object of the invention is to present an improved flux pump arrangement providing an operational method for applying a voltage across the magnet coil system, which is constant throughout all cycles of the flux pump.